In some communications systems, digital components are used to create the outgoing signal or to deconstruct the incoming signal. This allows the system to perform its function cheaper and with a more efficient use of power. Further, the use of digital components allows the vagaries of analog components to be discarded. Thus, the further use of digital components allows communications to be performed with lower cost and/or lower component counts than used in wholly analog systems, and without huge amounts of analog expertise required.
In some cases, a signal having certain phase or frequency characteristics can be input into a driver stage to be amplified. This is the case in frequency modulation, as well as that employed in combined amplitude modulation/phase modulation systems. In these cases, the conventional systems typically transform the digital signal into an analog signal by outputting the digital data (or derivation thereof) an output of a voltage controlled oscillator (VCO). In this method, the output of the system is then tested against a reference, and the error signal is applied to a loop filter which produces an appropriate correction signal for the system. In this case, control apparatus is usually accomplished with a phase lock loop or a frequency control loop.
However, the signal to noise ratio (SNR) of the system will fall drastically when the clocking speed of the loop filter (or other digital component that drives the VCO) is an integer divisor of the output signal. Thus, when the system operates at output rates that are harmonics or sub-harmonics of the clocking rate of the driver mechanism in a phase lock loop or frequency control loop, that operation can be problematic in a typical communication system. The most pronounced effects on the output of such a system is a frequency that is an integer multiple of the output frequency. In this case, this mode of operation leads to SNR (or other indication of noise) indicating much greater adverse operating characteristics than at the other sub-harmonics.